The subject matter of this application is related to the subject matter of British Application No. GB 0008268.5, filed Apr. 4, 2000, priority to which is claimed under 35 U.S.C. xc2xa7 119 and which is incorporated herein by reference.
1. Field of the Invention
This invention relates to the winding of coils used in electrical machines, particularly those machines in which one coil does not overlap an adjacent coil.
2. Description of Related Art
Electrical machines make use of flux flowing in a magnetic circuit to convert energy from one form to another. A transformer can be used to convert energy from one voltage level to another. Rotating electrical machines convert electrical energy to mechanical energy when acting as a motor, and mechanical energy to electrical energy when acting as a generator. All of these machines require electrical windings which carry either excitation or load current. These windings are typically composed of one or more coils of conducting wire (e.g. copper or aluminum) which is coated with insulating enamel to provide electrical insulation between adjacent turns of the coil.
Because of the wide variety of machines in existence, many different types of windings are required and different techniques are required for the satisfactory manufacture of the differing shapes of coils. For relatively small machines, round wire is normally used and, because production volumes are often high, automated methods are known for producing the required coils. Typically these coils have large numbers of turns, e.g. 100 turns or more, and the wire is relatively fine, e.g., less than 0.5 mm in diameter, so the coils are xe2x80x9crandomxe2x80x9d or xe2x80x9cmushxe2x80x9d wound, i.e. the position of any one turn within the coil is not predetermined. As the machine size increases, the number of turns required falls and the wire cross-section has to increase to carry the larger currents required. This leads to the adoption of a xe2x80x9clayeredxe2x80x9d winding, where the position of each turn in the coil is controlled to give the highest possible amount of conductor in a given cross-sectional area. FIG. 1 shows a cross-section of such a coil where the turns are arranged in a xe2x80x9chexagonal packingxe2x80x9d format. This construction is typically adopted for wire diameters between 1 and 5 mm.
However, for larger machines, which generally require fewer turns in the coil, the best use of the available space requires rectangular section wire. Such wire, while capable of yielding a very high density of wire in a given space, is much more difficult to wind and normally involves a labor-intensive process. A cross-section of a coil wound with rectangular strip wire to give a close-packed format is shown in FIG. 2.
The required profile of the coil is very dependent on the type of electrical machine. In machines which employ a rotating wave of magneto-motive force (mmf), a xe2x80x9cdistributedxe2x80x9d winding is typically employed, in which each coil spans several slots in the stator and therefore overlaps one or more adjacent coils. In order to accommodate the coil overhang (defined as the portion of the coil outside the active length of the stator core), a diamond shape is often used, as illustrated in FIG. 3. Such coils and their methods of production are discussed in many textbooks, e.g. xe2x80x9cThe Performance and Design of Alternating Current Machinesxe2x80x9d, M. G. Say, Third Edition, published by Pitman in 1958, Chapter 10, pp. 196 to 216, which is incorporated herein by reference.
In other types of machines, a coil spans only a single tooth or pole. These machines often have stators with parallel-sided poles, so it is conventional to wind the coils on a former and subsequently mount the coil on the pole with a suitable insulation system between the pole and the coil. Such coils are found in DC machines (in the field winding) and in switched reluctance machines. The side and center section views of a typical coil profile are shown in FIG. 4, in which, for clarity, the coil is shown as composed of only four turns. Also shown in FIG. 4 is the former on which the coil is wound. The former has a body 42 (which typically mimics the dimensions of the pole to which the coil will eventually be mounted) and sides 43 and 44 to maintain the correct width of the coil during winding. The thickness of the body of the former, dimension T in FIG. 4, is generally equal to the width of the pole, with an adjustment for the thickness of the insulation around the pole, if necessary. The width between the sides of the former, dimension W in FIG. 4, is generally a little less than the length of the pole.
The cross-sectional view of the coil in FIG. 4 is schematic in that it shows the coil as being close-packed at all points. However, as is well known to those skilled in the art, there are always voids in the coil at the point where a transition is made from one layer to another. This will now be explained by reference to FIG. 5, where the coil of FIG. 4 is shown partly made. The coil is started by bringing the wire in through the side of the former at the point marked xe2x80x9cStartxe2x80x9d and is bent through 90xc2x0 to begin the first turn. The cross-section marked 51s denotes the start of the first turn, at the top of the former. The wire is led down behind the former and appears at the bottom, as denoted by the cross section 51m, the middle of the first turn. This turn is completed by taking the wire, here marked as 51xe2x80x2 back to the top of the former, but it must be displaced laterally to lie beside 51s. Thus, it is led diagonally to the position shown as 51f, the finish of the first turn. A triangular void 54 is produced between 51xe2x80x2 and the side 44.
The point 51f is also the start of the second turn, in which the wire is led down behind the former, appearing in the position marked 52m, i.e. the middle of the second turn. To complete this turn, the wire, now marked as 52xe2x80x2, must be led to the top of the former, but must lie on top of the portion marked 51xe2x80x2, which, at the top of the former, is already occupying the space adjacent the former body. Raising the portion 52xe2x80x2 leaves a triangular void beneath it, bounded by the portion 51xe2x80x2, the side piece 43, the former body 42 and the portion of wire 52xe2x80x2. The portion marked 52xe2x80x2 completes the second turn at the cross-section marked 52f. It will now be seen that the turns lie parallel and adjacent each other behind, below and on top of the former. However, on the long side of the former carrying the second half of each turn, there are triangular voids between the sides of the former and the first and last turns, respectively. Because FIG. 5 is drawn for simplicity, there are only two turns per layer and the triangular void is exaggerated. Nevertheless, it illustrates the situation in which, by transposing between layers along the long side of the coil, this side occupies a greater height than the other side (which is only the height of the single turn).
It is well-understood by the skilled person that the transposition in the next layer of the coil runs diagonally in the opposite direction (i.e. the wire portion corresponding to 51xe2x80x2 will run from lower left to upper right), and that the third layer is laid down in the same direction as the first, etc. A complete coil is therefore found to have the transpositions running in alternate directions in alternate layers and to have triangular voids at opposite sides of the coil running in alternate directions in these alternate layers. In a coil with many layers, the side with the transpositions will typically be between 1.3 and 1.6 times the thickness of the other side. This increase in coil side thickness can give rise to a major difficulty when mounting the coils on the poles of the stator, and is often the limiting feature in the design of the winding.
In order to alleviate this difficulty and to provide a coil in which the long sides have the same cross-section, it is known to make the transpositions between layers occur in the short side of the coil (i.e. in the top, circular portion of the former shown in FIG. 5). In this case, the start of the coil is kept as far to the back of the former as possible and the second long side of the first turn (corresponding to 51xe2x80x2) is kept parallel to the side 44 of the former, only being allowed to traverse diagonally across after it passes onto the circular portion. While this gives the desired parallel sides down the length of the coil, the distance allowed for the transposition is very short and the wire has to be very accurately formed to maintain the correct dimensions. This involves a great deal of manipulation of the wire by the coil winding person which greatly increases the cost of the coil and often leads to damage of the insulation on the wire. During manipulation of the wire, the tension which is conventionally exerted on the wire to produce a tightly packed coil has to be released, often causing the turns already formed to spring out of position and add to the difficulty of manufacture.
There is therefore a need for a former which can economically produce coils with the transposition in the short side of the coil without damaging the wire insulation.
Embodiments of the invention provide guide channels for each turn of the winding on the post or posts mounted on the body of the former. In this way, the turns are entrained to follow the correct path. The side walls between the channels on the former define an aperture so that a turn in one channel can traverse across to start the next turn at a predefined point. Embodiments of the invention provide means for quickly and accurately laying down layered windings, which has been needed, but not available, for many years.